1. Field of the Invention
This invention relates to a radio communication system, a receiver, a receiving method, a transmitter, a transmitting method, and a device for and a method of calculating delay times to perform multi-carrier transmission in a multi-path environment such as a room wherein reflected waves and delayed waves as well as direct waves are propagated. Specifically, this invention relates to a radio communication system, a receiver, a receiving method, a transmitter, a transmitting method, and a device for and a method of calculating delay times to perform multi-carrier transmission by allotting data to be transmitted to carriers of different frequencies in order to cope with delay distortion.
More specifically, this invention relates to a radio communication system, a receiver, a receiving method, a transmitter, a transmitting method, and a device for and a method of calculating delay times to perform multi-carrier transmission by providing guard intervals between symbols to be transmitted in order to prevent interference between symbols. Furthermore, this invention relates to a radio communication system, a receiver, a receiving method, a transmitter, a transmitting method, and a device for and a method of calculating delay times to perform multi-carrier transmission by providing guard interval periods, which do not raise the transmission power, between symbols to be transmitted in order to prevent interference between symbols and among carriers.
2. Description of Related Art
With computers of ever-rising performance, there is a growing tendency for us to connect computers to build LAN's (Local Area Networks) in order to share data, information, and files and exchange data, information, and pieces of mail.
In a conventional LAN, computers are connected with optical-fiber cables, coaxial cables, and twisted-pair cables. Thus, such a conventional LAN requires complicated wiring of such cables; accordingly, it cannot be built easily. Besides, such a conventional LAN is not convenient because the ranges of movement of its component devices are limited by the length of cables.
The wireless LAN has lately attracted considerable attention, wherein components devices such as PC's can be moved relatively easily because they are free of connecting cables.
As the operating speed of wireless LAN's has increased and their costs has lowered recently, the demand for them has increased remarkably. Besides, the introduction of the personal area network (PAN) is considered. The PAN is a small-scale wireless network of electronic devices around us.
If a wireless network is built in a room, there is created a multi-path environment where each receiver receives direct, reflected, and delayed waves at the same time. The multi-path transmission causes delay distortion (frequency-selective fading), which causes errors in communication and interference between symbols.
One of the measures against delay distortion is the multi-carrier transmission system. According to the system, data are allotted to two or more carriers of different frequencies and transmitted; therefore, the bandwidth of each carrier is narrow and, hence, each carrier is less affected by frequency-selective fading.
Adopted in, for example, IEEE 802.11a, one of the wireless LAN standards, is the OFDM (Orthogonal Frequency Division Multiplexing) system, one of the multi-carrier transmission systems. According to the system, the frequencies of carriers are set so that they will lie at right angles to one another. Serially transmitted signals undergo serial-parallel conversion every symbol period, or cycle, slower than the data-transmission rate and are outputted. The outputted signals are allotted to the carriers. The signals of each carrier undergo amplitude and phase modulation. The signals of the carriers undergo inverse FFT to become signals on the time axis, the carriers lying at right angles to one another on the frequency axis, and are transmitted. Received signals undergo FFT to become signals on the frequency axis. The signals of each carrier undergo demodulation corresponding to the above modulation of signals of said carrier. The signals of the carriers undergo parallel-serial conversion to become serial signals.
According to the OFDM transmission system, the length of symbols can be increased by using two or more subcarriers lying at right angles to one another; therefore, the OFDM transmission system is suitable to multi-path transmission. However, there is the problem that if there are multi-path components, delayed waves interfere with the next symbol, causing interference between symbols. Besides, there occurs interference between subcarriers (interference between carriers), too, deteriorating the characteristics of reception.
On the other hand, there is available a method of providing guard intervals between symbols to be transmitted in order to prevent interference between symbols. Namely, guard signals such as guard intervals or guard bands in accordance with prescribed guard-interval or guard-band size and times are inserted between symbols to be transmitted.
It is also common to repeatedly transmit part of the signal to be transmitted in each guard interval period (see, for example, non-patent literature 1). By inserting a repetitive signal into guard interval periods, multi-path propagation (propagation of waves of multiple reflection) smaller than the size of guard intervals can be absorbed and interference between subcarriers can be prevented. Thus, the fatal deterioration of the quality of reception is prevented. Besides, inserting a repetitive signal into guard intervals brings about advantages such as synchronization of times and periods, or cycles, of symbols. If a repetitive signal is not inserted into guard intervals, the bit-error rate lowers (see, for example, non-patent literature 2).
On the other hand, if a repetitive signal is inserted into guard interval periods, the repetitive signal is removed at a receiver; accordingly, the repetitive signal transmitted does not contribute as signal power at the receiver. Therefore, inserting a repetitive signal causes the electric power necessary for transmission to increase.
Besides, inserting a repetitive signal causes the length of symbols to be transmitted to increase; accordingly, interference among carriers occurs, increasing the electric power necessary for transmission per unit frequency. If the electric power necessary for transmission per unit frequency is restricted by law, the transmission power has to be reduced by the above increment beforehand, causing the signal-noise ratio to deteriorate.
To save electric power, a null signal instead of a repetitive one may be inserted into guard intervals. In this case, the problem of the decline of the bit-error rate can be solved by using, for example, an equalizer (see, for example, non-patent literature 3 and 4). In this case, however, the adoption of an equalizer causes the circuit of the system to become complex and the cost of the system to increase.
[Non-Patent Literature 1]
    “Digital Broadcasting” by M. Shiomi et al. (Ohmsha 1998)[Non-Patent Literature 2]    “On the Use of a Cyclic Extension in OFDM” by R. Morrison et al. (0-7803-7005-8/$10.00 IEEE, 2001)[Non-Patent Literature 3]    “Performance Analysis of a Deterministic Channel Estimator for Block Transmission Systems with Null Guard Intervals” by S. Barbarossa et al.[Non-Patent Literature 4]    “Cyclic Prefixing or Zeor Padding for Wireless Multicarrier Transmissions?” by B. Muquet et al. (IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 50, NO. 12, DECEMBER 2002)